In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 by Brownstein entitled "Apparatus and Method For Controlling A Thermal Printer Apparatus," issued Nov. 4, 1986, the disclosure of which is hereby incorporated by reference.
Dye-receiving elements used in thermal dye transfer generally comprise a polymeric dye image-receiving layer coated on a support. Supports are required to have, among other properties, adequate strength, dimensional stability, and heat resistance. For reflective viewing, supports are also desired to be as white as possible. Cellulose paper, synthetic paper, and plastic films have all been proposed for use as dye-receiving element supports in efforts to meet these requirements. Recently, microvoided films formed by stretching an orientable polymer containing an incompatible organic or inorganic material have been suggested for use in dye-receiving elements. U.S. Pat. No. 4,778,782 of Ito et al., for example, discloses supports comprising a microvoided film obtained by stretching a translucent plastic film containing fine fillers such as clay or talc. By this stretching, bonds between the polymers and fillers in the film are destroyed, whereby microvoids are considered to be formed in the film. The microvoids lower the density of the film and also make it appear white and opaque. European Patent Application 0 322 771 discloses dye-receiving element supports comprising a polyester film containing polypropylene and minute closed cells within the film formed upon stretching.
A problem exists with the microvoided supports discussed above, however, in that it is difficult to manufacture films with a high degree of microvoiding. A high degree of microvoiding is desirable as this increases the heat insulating property of the support, and thereby the thermal efficiency of the dye transfer. EP 0 322 771 Comparative Example 4, for example, shows that a high degree of microvoiding in polyester/polypropylene stretched films, as evidenced by a relatively low specific gravity, results in poor mechanical strength and frequent breakage of the film during stretching. The lowest apparent specific gravity for an operable film in EP 0 322 771 is 0.71 (Example 2).
It would be desirable to provide a dye image-receiving element for thermal dye transfer with a manufacturable microvoided support which would provide superior thermal efficiency.